prescribing for astigmatism
Induced Astigmatism with Silicone Hydrogel Lenses
BY WALTER L. CHOATE, OD, FAAO
In my 20 years of practice, satisfying the visual needs of my astigmatic patients remains a significant challenge. Providing just the right amount of correction can make the difference between a truly happy and enthusiastic patient and one who is miserable.
Experienced fitters have come to expect some degree of change in corneal shape and geometry with long-term contact lens wear. Dr. Jerry Legerton once said that "you cannot not change the cor-nea when you fit contact lenses."
This change in shape can be positive, as with myopia control with GPs or myopia reduction in corneal refractive therapy. It can also be significant and not affect visual acuity, as with the inferior corneal steepening seen with most toric lens fits.
Silicone Hydrogel Lenses
Having worked with silicone hydrogel contact lenses as a continuous wear modality, I have come to understand that due to their high modulus, these lenses can, with some corneas, cause significant micro-irregular astigmatism in the central 3mm to 5mm of the cornea. This geometry change can be dramatic in patients who have highly malleable corneas, with the change occurring in a few weeks to a few months.
This is one reason why patients who wear silicone hydrogels should receive regular and diligent follow-up care. I have found that patients who are most susceptible to vision-affecting corneal changes seem to exhibit these changes within the first month of continuous wear. Patients with this problem present with reduced vision with their contact lenses and, more importantly, reduced vision with their spectacles after the lenses are removed. A clinical finding of reduced visual acuity with the lenses in place and a reduction of one to two lines in best-corrected visual acuity when the patient is refracted supports this. Corneal topography confirms the diagnosis, with a general molded appearance to the map and central cor-neal power variation of between 2.00D and 5.00D.
Figure 1. Induced astigmatism with silicone hydrogel lens
Figure 1 shows the cornea of a patient after silicone hydrogel lens wear, with resultant distorted appearance. This patient had been refit from a low modulus lens to a silicone hydrogel, and adapted well initially with great comfort and vision. The corneal distortion occurred in the first one to two months and did not return to baseline for almost nine months. This is the most severe case that I have observed; most cases return to baseline in two to four weeks. Recently, to speed corneal rehabilitation, I have in some cases refit the patient in a more loosely-fitting silicone hydrogel for daily wear until return to baseline occurs.
Expect corneal geometry changes when fitting contact lenses. Severe molding occurs in very few cases, but since those cases have been observed, be careful to educate patients about this potential adverse event.
Prescribe silicone hydrogels only after a thorough diagnostic fitting. Empirical fitting may result in tight fits with poor draping and a greater risk of adverse corneal geometry changes.
Dr. Choate is in private practice in Nashville, TN, a charter member of the AOA Contact Lens Section and an adjunct faculty member at Southern College of Optometry and University of Houston College of Optometry.